The present invention relates to a communications method in a system having a plurality of transmitters each suitable for transmitting a radio signal, and a receiver situated remotely from the transmitters to receive and process the signals transmitted by the transmitters, where the data to be transmitted is subdivided into N-bit symbols. The invention applies in particular to a system for remotely reading meters.
Systems are known having a plurality of transmitters transmitting radio signals that are received by a receiver that is situated remotely. In particular, U.S. Pat. No. 4,799,059 describes a remote reading system in which, after receiving a xe2x80x9cwakeupxe2x80x9d signal transmitted by the receiver, each transmitter transmits a radio signal including data relating to a water, gas, or electricity meter. The data comprises in particular a preamble, an identity number, a fraud parameter, values for measured parameters, and the type of measuring instrument.
According to the teaching of that document, the data to be transmitted is encoded using a Bose, Chaudhuri, Hocquenghem (BCH) error-correcting code which is applied to all or part of the preamble used, in particular by the receiver to ensure that the message it has received does indeed come from a transmitter of the system. The data is presented in the form of bits, and is encoded by a Manchester encoder which transposes the string of bits into a string of chips that are more suitable for transmission.
In above-mentioned U.S. Pat. No. 4,799,059, the number of bits allocated to data concerning the measurement instrument and to the measured parameters is equal to fifty-four, to which five bits are added that are reserved for possible growth of the message. With preamble sequences that are twenty-one bits long and a sixteen-bit long sequence generated by the BCH error-correcting code, the total number of bits in a message is equal to ninety-six.
However, it is advantageous to be able to lengthen the message that is to be transmitted, and thus increase the number of bits making it up. It may be desired to add data making it possible to provide better management of received messages, for example information about the location of the measurement instrument, or indeed to improve the precision of the measured parameters.
However lengthening the message to be transmitted raises a problem of degrading system performance, due in particular to an increase in the number of collisions between messages from different transmitters. When two messages from two different transmitters are received simultaneously by the receiver, at worst the receiver cannot process the messages at all, and at best it must process messages that include errors.
The present invention relates to a method of communicating data in a system having a plurality of transmitters, each suitable for transmitting a radio signal, and a receiver situated remotely from the transmitters to receive and process the signals transmitted by the transmitters, the data to be transmitted being subdivided into N-bit symbols and enabling the probability of collision between two frames coming from different transmitters to be reduced, other things being equal, and in particular for equal numbers of data bits. In other words, the invention makes it possible to decrease the occupancy of a radio channel. According to the invention, the invention consisting in:
giving a numerical value to each symbol; and
compressing the data using a chip encoding technique in which the values of the symbols are transposed to an encoded value having x digits in a base B, each digit subsequently being represented in the form of n chips, the base B, the number n of chips, and the number x of digits for an encoded value satisfying the following conditions:
Bxe2x89xa62n and 2Nxe2x89xa6Bx
xe2x80x83so that the total number of chips x.n representing the coded value is less than 2.N.
In a preferred implementation, the digits forming the encoded value are represented by equal numbers of 1 chips and of 0 chips. That makes it possible to avoid any DC components coming from the string of transmitted chips, by obtaining a mean value of zero from the distribution of value 1 chips and value 0 chips. The performance of the system, and in particular the performance of signal reception is thus improved.
In a particular implementation, each symbol has five bits, the base B is base 6, and the number n of chips is equal to four. Under such circumstances, a symbol encoded in the form of chips using the teaching of the prior art would be transmitted on ten chips, whereas in accordance with the invention, it is transmitted on eight chips.
In a particular implementation, the digits of base 6 are selected, in increasing order, to be equal to 1, 2, 3, 5, 6, and 7. This particular selection simplifies implementation of the binary conversion that generates the chips to be transmitted.
In a particular implementation, the processing of the signals received by the receiver includes a step of matched filtering.
In a preferred implementation, an error-correcting code is applied to the data to be transmitted. Using such an error-correcting code makes it possible to decrease the message error rate (MER), i.e. the ratio between the number of messages that are wrongly received to the total number of messages received during a given period of time. Furthermore, using an error-correcting code increases the probability of success concerning reading data after processing. Communication reliability is thus increased. This makes it possible to reduce the rate at which a given transmitter transmits a message. Consequently the lifetime of the batteries that may be required for transmitter operation is increased, and the number of collisions is also decreased. Advantageously, the error-correcting code is a Reed-Solomon code.
In a particular implementation, the Reed-Solomon code is defined in the Galois body GB(32).
Advantageously, the Reed-Solomon error-correcting code is a short code having the following parameters [S=21, K=16, d=6]. S represents the number of symbols processed; K represents the number of symbols to which the Reed-Solomon error-correcting code is applied; and d is the minimum code distance in the Galois body under consideration and is equal to Sxe2x88x92K+1. In the Galois body GB(32), S can be equal at most to thirty-one. Using a short code avoids uselessly lengthening the transmitted frame while conserving performance compatible with the intended application.
In an advantageous implementation, the processing of the signals received by the receiver includes a step of giving a reliability index to each symbol during a data decompression step.
In theory, applying a Reed-Solomon error-correcting code makes it possible to correct a maximum number of errors equal to E[(dxe2x88x921)/2], where E is the integer portion operator and d is the minimum code distance in the Galois body under consideration. In the particular case of a short code having the parameters [21, 16, 6], the number of errors that can be corrected is equal to two. However, as explained in greater detail in the description below, by using reliability indices, it is possible to correct up to five errors.
In a preferred implementation, a preamble is added to the beginning of a frame of encoded and compressed data, the preamble comprising a signal level sequence and a synchronization sequence. The synchronization sequence of the preamble is of the Turyn type with aperiodic correlation. Preferably, this synchronization sequence is 000000001110010101100100.
In a particular implementation, the radio signals are transmitted continuously by the transmitters in the form of bursts transmitted successively with a pseudo-random period. Under such circumstances, the system is said to be a xe2x80x9cone-wayxe2x80x9d system in that the receivers are not suitable for transmitting signals, and the transmitters are not suitable for receiving signals.
The method of the invention is advantageously applied to a system for remotely reading meters, which meters may, for example, be water, gas, electricity, or heating meters.
In a particular implementation, the data to be transmitted is subdivided, in this order, into: an identity field; a category field; a fraud field; a register field; a network field; and a growth field.
In another particular implementation, the data to be transmitted is subdivided, in this order, into: an identity field; a category field; an end field; a channel field; a growth field; and a data field.